1. Field of the Invention
The present invention relates to a lamp light controller for controlling, by way of example, a dashboard illumination lamp in motor vehicles.
2. Description of the Related Art
As a light control let for a dashboard illumination lamp, there is known one of the type that a PWM (Pulse Width Modulated) wave is used as a lamp driving current, and a duty ratio of the PWM wave is adjustable by a user for changing the illumination intensity of the lamp.
FIG. 4 shows a schematic configuration of such a light controller for lamps.
In FIG. 4, a PWM controller 100 generates a PWM wave which is boosted in its amplitude by a charge pump circuit (doubler circuit) 300 and then supplied to an N-channel MOS FET 400. The MOS FET 400 is turned on and off in accordance with the PWM wave supplied thereto, whereby a driving current is intermittently supplied to a lamp 500 for lighting it up.
The intensity of light emitted from the lamp 500 depends on a duty ratio of the driving current intermittently supplied. The PWM controller 100 includes a variable resistor 200 which can be operated by a user. By changing a resistance value of the variable resistor 200, the duty ratio of the PWM wave output from the PWM controller 100 is changed. Accordingly, upon the user operating the variable resistor 200, the duty ratio of the driving current for the lamp 500 can be changed to vary the illumination intensity of the lamp 500.
FIG. 5 is a circuit diagram showing a practical configuration of the light controller for lamps shown in FIG. 4.
In FIG. 5, a comparator 101, a capacitor 102, a diode 105 and resistances 103, 104 jointly serve as a ring oscillator, as a triangular wave oscillator 100A, which cooperates with a comparator 105 to constitute the PWM controller 100 (FIG. 4). In response to an output level of the comparator 101 being alternately changed to "H" (high level) and "L" (low level), the capacitor 102 is charged and discharged to thereby produce a triangular wave A. This triangular wave A is applied to a non-inverting input of the comparator 106, whereas a reference voltage B obtained from the variable resistor 200 is applied to an inverting input of the comparator 106. The variable resistor 200 comprises a fixed resistance 201, a rotary switch 202, and a resistance 203 providing a resistance value depending on a position of the rotary switch 202. A voltage resulted from the resistances 201, 203 dividing a source voltage+Vcc becomes the reference voltage B. The comparator 106 produces a PWM wave C which takes
"H" when level of the triangular wave A.gtoreq.reference voltage B, and PA1 "L" when level of the triangular wave A&lt;reference voltage B. PA1 an inverter for inverting a rectangular wave voltage output from the multivibrator, and PA1 a P-channel MOS FET to a gate of which the rectangular wave voltage output from the inverter is applied, PA1 the MOS FET having a source to which a source voltage is applied and a drain to which a lamp is connected, the MOS FET being on/off controlled with the rectangular wave voltage. PA1 an inverter for inverting a rectangular wave voltage output from the multivibrator, PA1 a P-channel MOS FET to a gate of which the rectangular wave voltage output from the inverter is applied, and PA1 a protective circuit connected in parallel to the MOS FET for protecting the MOS FET against an overload current, PA1 the MOS FET having a source to which a source voltage is applied and a drain to which a lamp is connected, the MOS FET being on/off controlled with the rectangular wave voltage. PA1 an inverter for inverting a rectangular wave voltage output from the multivibrator, PA1 a P-channel MOS FET to a gate of which the rectangular wave voltage output from the inverter is applied, PA1 a source voltage applied to a source of the MOS FET, and PA1 a lamp connected to a drain of the MOS FET, PA1 the MOS FET being on/off controlled with the rectangular wave voltage. PA1 a P-channel MOS FET to a gate of which a rectangular wave voltage output from the multivibrator is applied, PA1 a protective circuit connected in parallel to the MOS FET for protecting the MOS FET against an overload current, PA1 a source voltage applied to a source of the MOS FET, PA1 a lamp connected to a drain of the MOS FET, and PA1 a driver circuit connected to the output side of the multivibrator for controlling driving and stop of the multivibrator, PA1 the MOS FET being on/off control led with the rectangular wave voltage. PA1 a multivibrator provided with means for making its oscillation frequency variable, PA1 an inverter for inverting a rectangular wave voltage output from the multivibrator, PA1 a P-channel MOS FET to a gate of which the rectangular wave voltage output from the inverter is applied, PA1 a protective circuit connected in parallel to the MOS FET for protecting the MOS FET against an overload current, PA1 a source voltage applied to a source of the MOS FET, and PA1 a lamp connected to a drain of the MOS FET, whereby the MOS FET is on/off controlled with the rectangular wave voltage.
When the rotary switch 202 is operated to change the resistance value of the resistance 203, the reference voltage B is changed, whereupon a duty ratio of the PWM wave C produced from the comparator 106 is varied.
During an "L" period of the PWM wave C produced from the comparator 106, a gate potential of the MOS FET 400 is lower than its drain potential and the MOS FET 400 is not turned on. In order to effect that the MOS FET 400 is turned on during an "H" period of the PWM wave C, the gate potential of the MOS FET 400 must be higher than the drain potential. The doubler circuit 300 is required to raise the "H" level of the PWM wave C so as to meet such a condition.
The doubler circuit 300 comprises a diode 309, a capacitor 302, and resistances 303, 304. When an output of the comparator 101 in the triangular wave oscillator 100A takes "L", the capacitor 302 is charged to+Vcc through the diode 309. At this time, the capacitor 102 in the triangular wave oscillator 100A is abruptly discharged through the diode 105 and the resistance 103, but this discharging is not so abrupt as to impede the charging of the capacitor 302 in the doubler circuit 300. When the output of the comparator 101 in the triangular wave oscillator 100A takes "H", a charged voltage of the capacitor 102 is raised to make the PWM wave C from the comparator 106 become "H". Added to a potential at this "H" of the PWM wave C is a charged voltage of the capacitor 302 through the resistance 303 so that the gate potential of the MOS FET 400 is raised to exceed the drain potential. Accordingly, the MOS FET 400 is turned on.
In such an arrangement, by operating the rotary switch 202 to change the reference voltage B, a duty ratio of the PWM wave C produced from the comparator 106 is changed and hence so is an on/off duty ratio of the MOS FET 400. As a result, the illumination intensity of the lamp 500 shown in FIG. 2 is varied.
In the above-mentioned related art, the doubler circuit 300 is required to effect on/off driving of the MOS FET. More specifically, a voltage applied to the illumination lamp 500 attached in dashboards of motor vehicles is 12 (v) and this voltage gives the drain potential of the N-channel MOS FET 400. Therefore, the source voltage+Vcc is set to 12 (v). To turn on the MOS FET 400, the gate potential must be made sufficiently higher than the drain potential. But since the comparator 106 is driven with the source voltage +Vcc, the resulting potential at "H" of the PWM wave C is +Vcc at maximum. Thus, if only the potential at "H" of the PWM wave C is simply applied to the gate of the MOS FET 400, the MOS FET 400 would not be turned on. For that reason, the doubler circuit 300 is provided to increase the voltage at "H" of the PWM wave C about twice the source voltage.
However, the doubler circuit 300 has accompanied problems of requiring a large physical space and increasing an size of the whole controller. Also, adding the doubler circuit 300 has increased the number of parts and the production cost.
Further, the above-mentioned related art requires comparators as essentials; i.e., the comparator 101 in the triangular wave oscillator 100A and the comparator 106 for generating the PWM wave. The fact that these comparators are each of a complicated circuit comprising many parts has also increased the number of parts, cost and size of the conventional controller.
Additionally, in the case where head lamps of motor vehicles are automatically lit up and off by detecting brightness in the surroundings, a switch is required for turning on and off a dashboard illumination lamp or the like upon switching of the head lamps at the same time.